1. Field of the Invention
The present invention relates to an image forming system.
2. Description of the Related Art
There has been performed a density correction in image processing of an image forming apparatus. The density correction includes, for example, printer γ correction.
The “printer γ correction” means adjusting the relative relation between an ‘input value’ such as the density or the luminance of an input image input into the image forming apparatus and an ‘output value’ such as the density of an image actually formed on a sheet of paper on the basis of the input image. The performance of the printer γ correction decreases the influences owing to the apparatus characteristics of image forming apparatus to enable the formation of an image having colors faithful to those of an input image on a sheet of paper. Incidentally, screen processing and the like can also be cited as influencing factors of the density correction besides the printer γ correction.
For example, Patent Document 1 discloses a technique of performing printer γ correction by using a scanner (see Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2005-303701). To put it concretely, a γ correcting patch is formed on a sheet of paper, and the sheet of paper on which the γ correcting patch is formed is read by a scanner. The data of the original γ correcting patch and the data of the γ correcting patch read with the scanner are compared to each other, and the density data for canceling the difference is obtained. The disclosed technique performs the printer γ correction by using the density data for density correction.
Moreover, also the following technique has been known. That is, the technique provides an image density control (IDC) sensor in the neighborhood of the intermediate transfer belt in an image forming apparatus in place of using a scanner to read a γ correcting patch. The technique produces a density conversion table or a density conversion function for converting a voltage value of the IDC sensor into a density voltage on the basis of the voltage value obtained by reading a γ correcting patch formed on the intermediate transfer belt with the IDC sensor and density values such as X, Y, and Z tristimulus values obtained by reading the γ correcting patch with a colorimeter or the like, and produces a printer γ correction table, indicating a relation between input values and output values, by using the density conversion table to perform printer γ correction.
Moreover, for example, Patent Document 2 (Japanese Patent Publication No. 4274031) describes a technique based on the consideration of the fact that the density of an image actually formed on a sheet of paper includes differences of the color and the density of the paper to be a backing. That is, the technique forms a γ correcting patch on a sheet of object paper that is used by a user apart from a sheet of standard paper, and reads the γ correcting patch with a scanner to generate a target gradation characteristic according to the object paper, and then produces γ correcting density data by using the target gradation characteristic. Because a plurality of target gradation characteristics is registered by the technique of the Patent Document 2, the technique requires a user's operation of selecting the target gradation characteristic according to a sheet of object paper at the time of performing printing on the sheet of object paper.
Now, an image forming system (serial tandem system) composed of a plurality of image forming apparatus connected in a paper conveying direction has recently been proposed.
If the printer γ correction according to a sheet of object paper as described in Patent Document 2 is performed in such an image forming system, a user needs to perform the operation of selecting a target gradation characteristic according to the sheet of object paper in each of the image forming apparatus, thus the operations are troublesome and inconvenient.